In general, the top face of a piston of a direct fuel injection diesel engine is formed flat, but a direct fuel injection diesel engine for which the top face of a piston projects in a pentroof shape is known from Patent Publication 1 below.
When the top face of a piston is formed flat, the strength of a squish flow heading from a squish area of an outer peripheral part of the top face of the piston toward a cavity recessed in a central part of the piston is uniform in the circumferential direction. However, with regard to a piston having a pentroof-shaped top face, since the angle formed by the top face thereof and a side wall of a cavity changes in the circumferential direction, the strength of squish flow is nonuniform in the circumferential direction, and uniform mixing of air and fuel is inhibited, thus giving rise to the problem of an increase in harmful exhaust substances. In the arrangement described in Patent Publication 1 above, the angle formed by the top face of the piston and the side wall of the cavity is made constant in the circumferential direction by changing the shape of the side wall of the cavity in the circumferential direction, thus making the strength of squish flow uniform.
Patent Publication 1: Japanese Patent Application Laid-open No. 63-16124